One goal of microelectronic manufacturing is to increase the number of transistors on a device and thereby increase its operation speed. However, with increased transistor density and speed, power consumption is also increased dramatically. The heat generated from the increased power consumption can raise the microelectronic device temperature dramatically and degrade circuit performance and reliability. Therefore, reducing the overall device operation temperature is of great importance for optimum device performance.
Furthermore, operation of the transistors in a microelectronic device may cause non-uniform heating of the circuit. Certain points on the device may generate more heat than others, thus creating “hot spots”. Without such hot spots, it may be possible to increase the average power dissipation of the device while maintaining a desired temperature of the integrated circuit, thus allowing it to operate at a higher frequency.
One way to reduce hot spots is to form a layer of diamond underneath a device substrate, since the high thermal conductivity of diamond enables the diamond layer to spread thermal energy laterally and thus greatly minimize the localized hot spots on the device. However, there are problems associated with forming a diamond layer on a substrate, such as a silicon wafer. One problem with depositing diamond films of sufficient thickness (about 50 to 200 microns) on a silicon wafer (and thereby forming a diamond coated silicon wafer) is that there is a significant difference in the coefficient of thermal expansion (CTE) between silicon and diamond. This difference in CTE can lead to wafer warping, which may preclude further processing or use of the formed diamond coated silicon wafer. This warping can cause either a compressive stress or a tensile stress in the diamond coated silicon wafer depending on the diamond deposition temperature.
However, for example, in the case of a diamond coated silicon wafer in compressive stress after diamond deposition, the warping can be controlled by mechanically and/or chemically introducing defects to a first side of the diamond coated silicon wafer, for example, by the use of a surface roughening process, such as a grinding process. Introducing defects to the first side of the diamond coated silicon wafer induces a tensile stress in the wafer that cancels out any compressive stress induced by the diamond deposition. Thus, by introducing defects to the first side of the diamond coated silicon wafer, the wafer may be “tuned” (i.e., roughened until the stress of the wafer is close to zero) so that a substantially planar (or flat) diamond deposited silicon wafer (i.e., a substantially planar diamond coated silicon wafer) may be obtained. A silicon device layer may then be bonded to a second side of the substantially planar diamond coated silicon wafer, upon which circuit fabrication may be performed.
Such a substantially planar diamond coated silicon wafer may also be utilized after circuit fabrication to increase the mobility of electrons in the silicon device layer, thus increasing the device speed. Removing the defects from the first side of the substantially planar diamond coated silicon wafer by utilizing a polishing process, for example, induces a tensile strain in the silicon device layer. This tensile strain stretches the crystal lattice of the silicon device layer, so that electrons encounter less resistance as they move through the crystal lattice of the silicon device layer. It is well known in the art that electron mobility values in such a strained silicon device layer increase with the introduction of such a tensile strain. Thus, the speed of a microelectronic device can be improved through the use of a strained silicon device layer.
The present invention provides for methods of diamond fabrication and structures formed thereby that improve the flatness and electron mobility of a diamond coated silicon wafer.